Modern information infrastructure requires constant expansions of transmission bandwidth to meet the need for high speed data transmission. Such need becomes increasingly imminent as the use of Internet to transmit a variety of data such as text, voice and video grows rapidly each year. As a consequence, the conventional coax cable system can no longer sustain this need due to a high transmission rate in the order of several tens of Gb/s is required for Internet transmission while the coax cable system is capable of providing a transmission rate only in the order of several tens of Mb/s. As an alternate, optical fiber, because its potential to achieve a high transmission bandwidth up to 100 Teraherz and to decrease transmission loss, is replacing coax cable as the building blocks for future high speed communication network.
Several fiber optic transmission systems have been proposed or developed. Among them, the Dense Wavelength-Division Multiplexing (DWDM) technology make it possible to raise the communication capacity for a single line fiber from about 2.5 Gb/s up to over 10 Gb/s. It allows several wavelength of light signals transmitting at one single fiber at the same time without wavelength interference, each wavelength carrying different information signals to different destinations. The DWDM technology therefore holds the promise of greatly expanding telecommunication infrastructure without additional construction.
DWDM may be suitably incorporated into a variety of optic fiber systems to provide aforementioned advantages. For instance, DWDM can be used in a unidirectional optical fiber system to multiplex a number of signals, each about 2.5 Gb/s, derived from input fibers at the transmitting end into a single optical fiber and subsequently demultiplex such signals from this single fiber optical line into a number of fibers carrying various channels to individual receivers at the receiving end. Similarly, DWDM can be used in a bi-directional optical fiber system in which DWDM is used to multiplex and demultiplex at both transmitting and receiving end. The lightwave transmission capacity per fiber depends mainly on the number of the light signals in the passband of the fiber optical amplifier known as Erbium-doped amplifiers. The bandwidth of the commercial Erbium-doped amplifiers is about 30 nm at two communication bands, 1260-1360 nm and 1500-1650 nm. The bandwidth of the light source is as narrow as 0.2 nm in DFB semiconductor laser. Most current DWDMs are capable of multiplexing or demultiplexing 4 (i.e., defined as a 1.times.4 DWDM), 8 (i.e., defined as a 1.times.8 DWDM) or 16 (i.e., defined as a 1.times.16 DWDM) optical signals located in different waveband to or from one fiber optical line. As such, DWDM is being used widely in all kinds of fiber optical system DWDM and becomes one of the key components in modem fiber optical systems.
A few Discrete Micro-Optic technologies have been successfully developed to construct DWDM such as Filter-DWDM using multi-layer interference filters, Grating-DWDM using holographic gratings and waveguide-DWDM using optical circuit waveguides. These devices are, however, disadvantaged by a limitation that the light signals transmitted thereof have to leave the optical fiber first, processing in free space and then reinput the optical fiber in operation. This has led to serious difficulties in device manufacturing and adjusting.
U.S. Pat. No. 5,491,764 using twist FBT technique to make a narrow band Wavelength Division Multiplexer ("WDM") capable of multiplexing or demultiplexing two light signals known as two-channel WDM. This prior art device, however, does not teach construction of a narrow band WDM capable of multiplexing or demultiplexing a plurality of light signals requiring more than two channels because such prior art WDM is disadvantaged by high transmission losses resulting in a poor wavelength isolation insufficient to provide multi-wavelength transmission.
There is therefore an apparent need for an apparatus and method of making a low loss, inexpensive DWDM capable of providing a reliable multi-wavelength transmission in a fiber optical transmission system.
Accordingly, it is an object of this invention to provide a low loss, inexpensive and reliable narrow band DWDM comprising a plurality of FBT multi-window Wavelength Division Multiplexers ("MWDMs") for high speed, multi-wavelength transmission.
It is a further object of this invention to provide a method of making such FBT MWDMs by heating and stretching two optical fibers to form a FBT MWDM.